The implied motion aftereffect changes decisions, but not confidence
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bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
The implied motion aftereffect changes decisions,
but not confidence
Regan M. Gallagher*, Thomas Suddendorf, & Derek H. Arnold
School of Psychology, The University of Queensland,
Brisbane, Australia.
Correspondence: regan.gallagher@uqconnect.edu.au
Abstract
Viewing static images depicting movement can result in a motion aftereffect: people
tend to categorise direction signals as moving in the opposite direction to the implied
motion of still photographs. This finding could indicate that inferred motion direction
can penetrate sensory processing and impact perception. Equally possible, however, is
that inferred motion impacts decision processes, but not perception. Here we test these
two possibilities. Since both categorical decisions and subjective confidence are
informed by sensory information, confidence can be informative about whether an
aftereffect likely results from changes to perceptual or decision processes. We therefore
leveraged subjective confidence as an additional measure of the implied motion
aftereffect. In Experiment 1 (implied motion), we find support for decision-level changes
only, with no change in subjective confidence. In Experiment 2 (real motion), we find
equal changes to decisions and confidence. Our results suggest the implied motion
aftereffect produces a bias in decision-making, but leaves perceptual processing
unchanged.
Key Words: Perceptual aftereffect, Decision-making, Sensory adaptation, Confidence.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
An outstanding question in perception research is whether our thoughts, desires,
emotions, or cognitions can change how our sensory systems operate. Usually,
perceptual aftereffects are quantified by recording subjective responses to a range of
stimulus intensities (e.g. the brightness of lights, or tone frequencies), and measuring
decision changes before and after prolonged and repeated exposures to a specific
stimulus (see Clifford, Webster, Stanley, Stocker, Kohn, & Sharpee et al., 2007 for a
review). Aftereffects are also typically constrained to a common sensory dimension,
such as when a moving adaptor influences the perceived motion of a test (Barlow & Hill,
1963). However, recent research has started revealing aftereffects in which test stimuli
are only conceptually related to the adapting stimulus. These studies offer an empirical
approach to determine whether high-level cognitions (that extract meaning) can
change perception.
One study conducted by Winawer, Huk & Boroditsky (2008) had participants adapt to
still photographs that implied motion, either leftward or rightward, or inward or
outward. In a second study (Winawer, Huk & Boroditsky, 2010), participants adapted to
a static grating stimulus, and were asked to imagine that it was moving. In both studies,
participants then judged the motion direction of a dynamic dot stimulus. In each case,
the adaptation phase (static images, or imagined motion) gave rise to a negative
aftereffect: participants were more likely to judge a (possibly ambiguous) stimulus as
having moved in the opposite direction. This pattern of results is broadly consistent
with the classic motion aftereffect (Barlow & Hill, 1963), and the authors concluded that
the adaptation task had directly impacted motion perception. While this interpretation
is intuitive, there is an equally plausible interpretation: viewing static images that imply
movement, or imagining movement, might engage motion-related cognitions that bias
categorical decisions, but leave the sensory processes underlying motion perception
unchanged.
Recent research suggests that subjective confidence reports can provide an important
additional source of information to help discern whether or not an aftereffect is likely
to have a perceptual basis (Gallagher, Suddendorf, & Arnold, 2018). When decisions are
impacted by sensory adaptation, both the decision and confidence functions canbioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
provide equivalent measures of the perceptual aftereffect. However, when decisions
change for reasons other than a perceptual change (e.g. due to a biased pattern of
response when inputs are ambiguous), the range of inputs that elicit low confidence in
categorical judgments can remain veridical, and a dissociation between decision and
confidence response profiles can emerge. In this scenario, the dissociation constitutes
evidence for the aftereffect being driven by decisional processes.
Subjective confidence can be helpful for diagnosing the origin of an aftereffect by
providing a second source of information that is directly informed by sensory evidence.
Moreover, confidence can be dissociated from categorical decisions when decision
biases change categorisation responses (see Gallagher et al., 2018). Measuring changes
in confidence, in addition to category responses, can thus help to distinguish changes
in perception from changes in decision-making. In the present study, we find evidence
that viewing still photographs depicting movement changes categorisations of
ambiguous inputs, but not confidence. On this basis, we argue that that the implied
motion aftereffect is unlikely to be a perceptual aftereffect.
Perceiving versus deciding
Biases in decision-making are usually accepted as the evidence for sensory adaptation,
but these can equally occur in the absence of perceptual changes (see Morgan et al.,
2011). Based on biased decisions alone, we cannot tell whether a given aftereffect has
resulted from changes to perception or from changes to a decision process independent
of perception (for greater discussion on this topic, see Morgan et al, 2011; Storrs, 2015;
Firestone & Scholl, 2016; Fritsche et al., 2017). To illustrate, Figure 1 depicts two
components of perceptual decision-making: sensory evidence and decisional criteria.
Sensory evidence consists of the physiological information made available to the brain
by the sense organs. Sensory evidence is typically considered to be contaminated by
Gaussian noise, so different sensory information can be encoded on a trial-by-trial basis,
even if people are repeatedly exposed to an unchanging physical input. Decision criteria
can be regarded as a boundary, or threshold, separating when people will decide
whether to classify a stimulus as belonging to one category or another (as moving to thebioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
left or right in this context). The interpretation of weaker stimuli (with encoded values
close to a decision boundary) can be shifted to either side of the criterion on a trial-by-
trial basis by sensory noise, by a changing decision criterion, or both. So, decisions made
about weak signals are often probabilistic. Stronger signals might be equally variable
from trial-to-trial, but variable decision or sensory processes will have less impact on
their interpretation, because the encoded value is more distant from the criterion.
Figure 1: A discrimination task with two binary responses: decisions, left or right, and
confidence, high or low. (top) A sigmoid function (S-curve) represents the sensory
signal-to-noise ratio (with greater noise implied by lighter shading) plus response
criteria (red dotted lines). As the stimulus intensity approaches an encoded value of 0,
the observer’s decisions become more probabilistic. The placement of decision criteria
impacts central tendency estimates for the decision distribution (d1 = unbiased, d2 = left-
response bias). (bottom) A Gaussian function (U-curve) representing confidence in
decision-making. As the stimulus intensity approaches 0, confidence decreases to its
minimum. Response criteria for confidence relate to the strength of evidence needed to
report confidence, which can encourage more or less reports of confidence, but do not
encourage a lateral shift of the confidence function (c1 = low-confidence threshold, c2 =
high-confidence threshold).bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
If an observer is required to choose between two alternatives when an input is
perceptually ambiguous, they may rely on higher-order cognitive (i.e. non-perceptual)
aspects of decision-making. One might simply guess, leading to an equal likelihood of
choosing left or right. Alternatively, they might adopt some other strategy that carries
greater bias toward one or another category. A decision strategy could manifest as either
a negative or a positive bias (or none), independent of sensory evidence and other post-
perceptual processes (see Yarrow et al, 2011). Moreover, a wilfully adopted decision bias
can change estimates of decision boundaries, without impacting on the precision of
perceptual judgments (Morgan et al., 2011) or the central tendency of the associated
distribution of confidence (Gallagher et al., 2018).
The Present study
A participant whose task is to imagine or infer movement while viewing stationary
objects might, when prompted, show a statistical preference for the unimagined
direction when test stimuli are ambiguous (Winawer et al., 2008; Winawer et al., 2010).
In this example, the observer could be indexing unchanged sensory evidence against a
different criterion when uncertain. Crucially, humans can report on whether they are
confident or guessing, which can provide important additional information about how
to interpret otherwise-ambiguous changes in categorical responses.
Changes in decision-making could occur without physiological changes in motion-
sensitive brain regions, due to a shift in decision criteria. This would alter the
measurement of maximal categorical ambiguity—the inflection point of a cumulative
gaussian function fit to binary categorical decisions. This measure is often referred to as
the point of subjective equality (PSE), and changes in this metric from baseline trials
are used to estimate aftereffect magnitudes. However, while a shift in a decision
criterion will alter decision-making, they might have no impact on the range of inputs
that elicit uncertainty. An additional important point is that both measures (decisions
and confidence) are informed by sensory encoding. So, having people report on
decisional confidence, in addition to committing to a categorisation, might reveal abioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
dissociation. Confidence, or expressions of uncertainty, could remain veridical in the
case where categorisations are biased solely by decisional processes.
In two Experiments we measure categorical direction decisions and confidence reports
to help determine whether the implied motion aftereffect likely results from changes to
perceptual or post-perceptual decision processes.
Aim and hypotheses
The aim of the current study is to test whether implied motion from still images changes
perception of real motion. We use confidence measures to provide additional diagnostic
information. In Experiment 1, we measure motion-direction decisions following
adaptation to a stream of still photographs depicting directional motion (either to the
left or right). In Experiment 2, we compare this implied motion aftereffect to a more
conventional motion aftereffect, using a serial dependence between successive real
motion signals. We hypothesise that changes to sensory processing will impact
categorical direction decisions and confidence reports equally. In contrast, we
hypothesise that aftereffects arising from post-perceptual (decision) processes will
selectively impact direction decisions—non-perceptual changes in decision-making
should produce a dissociation between decision and confidence responses.
Experiment 1
Method
Participants
All participants (30 for each Experiment) were recruited from the University of
Queensland’s Psychology department. Sample sizes of 30 were set for all Experiments,
as these are comparable to samples used in the original studies of the imagined and
implied motion aftereffects. Participants were drawn from a first-year student pool, who
received course credit for their participation. All were naïve as to the purpose of the
experiments.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
Ethics
Ethical approval for all experiments was obtained from the University of Queensland’s
Ethics Committee, and experiments were conducted in accordance with committee
guidelines. Each participant provided written informed consent, and were aware that
they could withdraw from the study without penalty at any point.
Materials & Stimuli
Stimuli were presented on a Dell LCD monitor (1024x768 pixels). All computers were
running Matlab software and the Psychophysics Toolbox (Brainard, 1997; Pelli, 1997).
All monitors had a screen refresh rate of 60Hz. Adapting stimuli (Experiment 1 only)
and test stimuli were presented within an aperture size of 300 x 175 pixels against a grey
background (RGB = 125,125,125).
Adapting stimuli were photographs taken from a Google Image search, after searching
for images that depicted fast movement along the horizontal plane. The implied motion
stimuli were then compiled into a set of 100 photographs, with an approximately equal
aspect ratio in portrait orientation. The dimensions of photographs were re-sized before
presentation, so that they were all equal in aspect ratio. Each photo was mirror-flipped,
so it could be used to depict both directions.
Test stimuli consisted of 100 dots rendered blue against a grey background. Each dot
was 1 pixel in size. Initially, dots were drawn at random locations within the aperture
window. Dot coherence values ranged from -30 (30% coherence leftward) through 0
(random motion) to +30 (30% coherence rightward). Test stimuli were set to one of 11
coherence values [-30 -20 -10 -6 -3 0 3 6 10 20 30], presented in a randomised order.
Coherently moving dots were selected at random on each frame, so no individual dot
could be tracked across the screen. Coherent motion was achieved by displacing
coherent dots left or right by one pixel on successive frames. All other dots were redrawn
at random locations.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
Procedure
Participants sat comfortably in a chair approximately 55 cm from the display, resting
their hands on the keyboard’s directional buttons while fixating a central cross-hair. If
there was an adaptation phase, the adapting stimulus was presented for 18 seconds on
the first trial for each of five blocks, and again on the middle trial when the implied
motion direction reversed. For all other trials the adapting stimulus lasted for six
seconds. Participants passively viewed the implied motion images without responding.
See Figure 3 for a representation of the task procedure.
After the adaptation phase, participants were presented with one of the 11 dot-motion
test probes. Tests were presented for one second before disappearing, leaving only the
fixation cue. Participants reported whether the test had appeared to be moving left (by
pressing the left arrow key) or right (by pressing the right arrow key). If participants
could not determine the test direction, they were instructed to make their best guess.
Once the direction judgment had been made on each trial, the fixation cue turned black,
prompting a confidence response. Participants indicated whether they felt high
confidence in their direction judgment by pressing the up arrow on the keyboard, or
low confidence (or that they were guessing) by pressing the down arrow. The fixation
cross turned white once the confidence response had been provided, and a new trial
started immediately. Each of the 11 stimulus values was tested five times per block, and
participants completed two blocks in total (once with a leftward adaptor and once with
a rightward adaptor) resulting in 110 individual test trials per participant. The initial
adaptor direction was randomised for each participant.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
implied motion
time
Adapt (6s)
Response
varied motion (1s) until response until response
+ +
Test Decision Confidence
0% - 30% (left or right) Left or Right High or Low
Figure 2: Experiment procedure. In Experiment 1, participants adapted to still
photographs that depicted motion to either the left or right (one direction for each
testing block). Adapting stimuli appeared for 18s on the first trial of each block, and on
the middle trial, and for 6s on all other trials. There was no adaptation period in
Experiment 2. Each trial then had a dot test probe. Tests were present for 1s, appearing
on the second frame after the adapting stimulus disappeared. A new trial began once
participants had recorded their direction decision (left or right) and reported their
confidence in their decision (high or low).
Data preparation
Categorical direction decisions were scored as a 1 if the person said the stimulus moved
to the right, or 0 if they said left. The response function approximated a Sigmoid
distribution when the proportion of rightward decisions is plotted as a function of
motion direction and coherence. The inflection point of the Sigmoid function (for an
unbiased observer) should approximate 0% coherence, indicating an equal probability
of choosing leftward and rightward motion when the stimulus has random physical
movements.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
Confidence responses were scored as a 0 if the person reported being confident in their
direction decision, or as -1 if they were not confident. This response distribution
approximated a raised Gaussian function (bell curve) when responses were plotted as a
function of motion direction and coherence. The peak of the confidence function is the
point of lowest confidence, which should be centred around 0% coherence for an
unbiased observer. Two aftereffect measures were taken for each participant in each
Experiment: one from the inflection point of decision responses, and one from the peak
of confidence responses.
Results
All t-tests reported below are two-way repeated-measures tests for equality of means.
All Bayes Factor analyses were estimated using JASP software (JASP, 2016) with a Cauchy
prior width of 0.707.
Implied motion aftereffect
Analyses for Experiment 1 showed that adaptation to still images depicting motion had
a robust impact on direction decisions. Results showed that decisions following leftward
adapting images (LPSE = 0.71; SD = 2.70) were significantly different from decisions
following rightward adapting images (RPSE = 2.50; SD = 4.70; t29 = 2.94, p = .006, Cohen’s
d = 0.54, 95% CI 0.15 – 0.91, BF10 = 6.56). However, implied motion adaptation did not
impact measures of confidence. The central tendency of confidence reports following
leftward implied motion adaptors (LCONF = 0.84; SD = 1.50) was not significantly
different to the central tendency of confidence reports following rightward implied
motion adaptors (RCONF = 0.78; SD = 2.29; t29 = 0.13, p = .896, BF10 = 0.20). We also
measured a dissociation between the effect of implied motion on decision (∆PSE = 1.79;
SD = 3.33) and confidence reports (∆CONF = -0.06; SD = 2.55; t29 = 2.50, p = .018, Cohen’s
d = 0.46, 95% CI 0.08 – 0.83, BF10 = 2.71). These data are depicted in Figure 3.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
a) b)
c)
Figure 3. Results of Experiment 1. (a) psychometric functions depicting (top) the
proportion of rightward responses and (bottom) the proportion of confidence
responses, both as a function of motion direction and coherence. Separate lines depict
direction of the implied motion adaptor. (b) average aftereffect magnitudes, as
measured by decision and confidence. (c) Individual data showing changes in decision
and confidence following implied motion adaptation. N = 30; error bars depict ± 1 s.e.m.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
Experiment 2
Method
The methodological details for Experiment 2 are identical to Experiment 1 except for the
following. One, participants sequentially responded to the test stimuli without an
adapting stimulus. Two, there were eight test values [-30 -15 -5 -1 +1 +5 +15 +30] rather
than the 11 test values in Experiment 1. There was no 0% coherence value, which allowed
us to analyse responses according to the physical direction of the previous test. Three,
participants completed 50 observations of each of the 8 test values for a total of 400
observations per observer.
Results
Rapid motion aftereffect
Analyses for Experiment 2 showed that the test motion direction on the previous trial
had a robust impact on subsequent direction decisions. Results showed that decisions
following leftward tests (LPSE = -1.02; SD = 2.85) were significantly different from
decisions following rightward tests (RPSE = 1.05; SD = 2.31; t27 = 3.41, p = .002, Cohen’s d
= 0.65, 95% CI 0.23 – 1.05, BF10 = 18.09). The previous test motion direction also impacted
measures of confidence. The central tendency of confidence reports following leftward
tests (LCONF = -0.95; SD = 2.23) was significantly different to the central tendency of
confidence reports following rightward tests (RCONF = 1.76; SD = 2.92; t27 = 3.38, p = .002,
Cohen’s d = 0.64, 95% CI 0.23 – 1.04, BF10 = 16.77). The measured effect of previous trials
was not significantly different for decision (∆PSE = 2.07; SD = 3.21) and confidence
reports (∆CONF = 2.71; SD = 4.24; t27 = 0.70, p = .488, BF10 = 0.25). These data are
depicted in Figure 4.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
a) b)
c)
Figure 4. Results of Experiment 2. (a) psychometric functions depicting, top, the
proportion of rightward responses and, bottom, proportion of confidence responses as
a function of motion coherence. Separate lines depict direction of the previous motion
stimulus. (b) average aftereffect magnitude as measured by decision and confidence. (c)
Individual data showing changes in decision and confidence following rapid motion
adaptation. N = 28; error bars depict ± 1 s.e.m.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
Discussion
Our results suggest that the implied motion aftereffect is the result of changes to post-
perceptual decision processes. Experiment 1 replicated the effect of viewing a stream of
still photographs implying directional motion. As predicted, and consistent with
previous research (Winawer et al., 2008), our results showed that participants were
more likely to report that tests were moving in the opposite direction relative to that
implied by still adapting images. The results of confidence responses, however,
suggested that the region of peak uncertainty was unchanged by adapting to implied
motion, and a dissociation between direction decisions and confidence reports was
observed.
Experiment 2 showed that the PSE and confidence measures were equally influenced by
the previous physical test motion. Tests on the current trial (trial n) were more likely to
be reported as moving in the opposite direction relative to the last test (trial n-1). The
probability that the current test stimulus would evoke low confidence was also
impacted by the previous test stimulus. Importantly, the shift in confidence reports was
in the same direction as the shift in decisions, and of equal magnitude. These results are
consistent with both responses (direction decisions and confidence) being informed by
a common source of information (perception), which was equally impacted by recent
sensory history.
Previous research shows that perceptual changes result in an equivalent impact on the
central tendency of both decision and confidence reports (Gallagher et al., 2018),
whereas changes in post-perceptual processes can change decision-making without
changing confidence. The pattern of results in Experiment 2 is consistent with evidence
for a perceptual aftereffect, whereas the decision aftereffect in Experiment 1 is consistent
with changes in non-perceptual processes.
Our results show that the degree to which implied motion (Experiment 1) and serial
dependence (Experiment 2) impact motion direction decisions was approximately equal
(both Cohen’s d effect sizes ~0.6). However, only exposure to real motion produced a
change in confidence reports. This finding is important in at least two ways. First, itbioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
shows that confidence measures are sufficiently sensitive to detect moderate sized
changes in decision-making, ruling out the possibility that confidence is not sensitive
enough to detect perceptual changes from viewing still photographs. Second, it shows
that aftereffects that elicit equal changes in categorical decision-making (compare the
decision aftereffect estimates of Exp1 and Exp2) can nonetheless be separated based on
how they impact another response (confidence) which also relies on sensory evidence.
These results point to the utility of confidence in the study of perceptual aftereffects.
A core finding in our serial dependence results is in agreement with previous research
showing that perceived motion can undergo rapid sensory adaptation (Kanai &
Verstraten 2005; Glasser et al., 2011), and that rapid negative motion aftereffects have an
equivalent impact on the central tendency of both decision and confidence reports
(Gallagher et al., 2018). While our serial dependence results showed a negative
aftereffect, many other serial dependencies have revealed positive aftereffects (Fischer
& Whitney; Cicchini, Mikellidou, & Burr; Fornaciai & Park, 2018), which are thought to
produce a more stable representation of our dynamic world, although it remains a
matter of debate whether this stability is a decision-level effect rather than a perceptual
experience (Fritsche et al., 2017). Judgements of confidence could help reveal whether
other serial dependencies likely have a perceptual basis.
Aftereffects induced by implying or imagining motion have previously been interpreted
as resulting from a top-down impact of cognition on motion perception (Winawer et
al., 2008, 2010; Pavan et al., 2011). This claim is bolstered by electrophysiological
evidence, that viewing photographs depicting implied motion activates some of the
same direction-selective cortical circuits as viewing real motion (Lorteije et al., 2006;
Lorteije et al., 2007). However, it is not clear from either set of evidence that motion
perception has been changed by cognition. Instead, different decisions could be
reached, and similar neural activations could be observed, by changes to decision
processes, even in the absence of changes to motion perception.
Although our data cannot determine why implied motion adaptation produces a
systematic negative aftereffect, previous research demonstrates that arbitrary decisionbioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
strategies can produce data consistent with either a positive or a negative aftereffect
(Morgan et al., 2011; Gallagher et al., 2018). One possibility is that people surreptitiously
perform a categorisation task whenever an unambiguous signal is encountered. This
could result in a bias toward categorising ambiguous inputs in the opposite category,
relative to the more recently viewed (adapting) direction signal. This describes the
frequency principle—a propensity to assign equal numbers of inputs to either category
when making dichotomous classifications (Parducci et al., 1960; Parducci & Wedell,
1986). So, if a given class of input is repeatedly ‘implied’ as an adapting signal, people
might compensate by assigning more weight to the less frequent category.
Conclusion
Our data suggest that the implied motion aftereffect reflects changes in decision-
making, independently of changes to sensory processes. This contrasts with the motion
aftereffect that changes both directional decisions and estimates of directional
uncertainty.
Authorship declaration
RMG and DHA designed the experiments. RMG conducted the experiments, analysed
the data, and wrote the manuscript. RMG, TS, and DHA revised and approved the
final manuscript. The authors declare no conflict of interest.bioRxiv preprint first posted online Dec. 17, 2018; doi: http://dx.doi.org/10.1101/498048. The copyright holder for this preprint
(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
It is made available under a CC-BY-NC-ND 4.0 International license.
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